Pressurised water nuclear reactors include a primary circuit in which pressurised water cooling the nuclear reactor circulates, this generally comprising several loops in each of which there is a steam generator which heats and vaporises the feed water through the exchange of heat between the pressurised nuclear reactor cooling water and the feed water to generate steam which is delivered to the nuclear power station's turbine. Steam generators for pressurised water nuclear reactors comprise an envelope of generally cylindrical shape with its axis vertical, within which is located a heat exchanger tube bundle and a substantially hemispherical water chamber comprising the lower part of the steam generator, below the envelope enclosing the bundle.
A thick tube plate located horizontally and perforated by systematically arranged holes in a vertical direction provides separation between the upper part of the steam generator and the water chamber. The tubes in the steam generator bundle are engaged and fixed into the holes in the tube plate by means of their extremities in a leaktight manner. The water chamber is bounded by both the hemispherical wall constituting the lower part of the steam generator envelope and the horizontal undersurface of the tube plate into which the holes for the passage and attachment of the tubes of the bundle open.
Each of the tubes in the bundle is arched at the top and has two straight branches whose extremities engage in the holes passing through the tube plate on either side of a partition plate separating the water chamber into two compartments each connected via a pipe to a primary circuit duct in such a way that the reactor cooling water enters the water chamber in one of the compartments in order to be distributed to each of the tubes in the bundle.
The pressurised cooling water circulating within the tubes of the bundle is collected in the second compartment of the water chamber so that it can be collected by a duct in the primary circuit connected to the second compartment of this water chamber.
In order that access may be gained to the water chamber the hemispherical envelope of this water chamber is traversed by an inspection opening or manhole in each of the two parts of the hemispherical envelope bounding one compartment of the water chamber.
On its underside the tube plate comprises a band forming an additional thickness, also known as a partition stub, with which the upper edge of the partition plate is aligned.
The partition plate is attached to the tube plate by a weld at each of its longitudinal edges.
While the nuclear reactor is in operation, the tube plate and the partition plate are subjected to mechanical and thermal stresses. These stresses also have an effect in the connecting areas between the tube plate and the partition plate, that is to say in the weld itself and above all in the areas adjacent to that weld.
After operation for some time these stresses can give rise to the appearance of cracks or incipient cracks in the connecting areas which need to be detected to prevent any leakage of pressurised cooling water into the inner space of the steam generator envelope to which feed water is fed so that they will not prejudice the integrity of the pressurised envelope should cracks propagate.
In the situation where cracks or incipient cracks are detected in these areas, a repair has to be made in order to avoid the risk of leaks or compromising the integrity of the pressurised envelope when the steam generator is brought back into operation.
These repairs are generally performed during scheduled shutdowns of the nuclear power station for recharging the reactor vessel with fuel assemblies.
These repairs may be carried out by operators working within the water chamber of the steam generator.
This technique has the disadvantage that operators have to work in a highly radioactive area containing deposits of activated materials which are carried into the water chamber by the nuclear reactor cooling water circulating in contact with the core's fuel assemblies. This being the case the doses received by operators may be large, even when work is carried out very quickly with each of the operators only being present in the water chamber of the steam generator for a very short time.
Many operators therefore have to carry out the work, so that the doses which they receive are limited.
In order to avoid the disadvantages associated with the need for operators to enter into the water chamber it is known that devices remotely controlled from outside the water chamber can be used to carry out inspection operations and to carry out various tasks, for example eliminating cracks or incipient cracks, or for any other work.
These remote controlled devices can reduce the time during which operators are present within the steam generator water chamber, or even do away with the need for operators to work within the water chamber.
For this purpose an anthropomorphic robot arm which is caused to enter the water chamber through the inspection opening and which includes an attachment portion attached to the wall of the water chamber in the vicinity of that opening is known.
Devices for working within a water chamber of a steam generator which incorporate a plate attaching the device to the undersurface of the tube plate and a turret which can be mounted on the attachment plate so that it can rotate about an axis perpendicular to the plate, that is to say a vertical axis perpendicular to the tube plate while the work is being carried out, are also known.
The device also comprises one or two telescopic arms which rotate as one piece with the turret in such a way that they can be caused to move in rotation about the axis of the turret in order to sweep through one or two annular fields.
The devices used hitherto are in general fairly bulky and extremely complex in their structure and require long and difficult operations, in particular for inserting and positioning them within the steam generator's water chamber.
Furthermore, these devices are generally provided for specific operations and are not intended to withstand large forces, particularly in the course of machining operations.
Also, fitting the device within the water chamber in an initial stage of the operation is a complex task which may require at least one operator to be present within the water chamber.